1. Field of the Invention
This invention relates to ion traps and to methods for fabricating ion traps.
2. Description of the Related Art
Conventional ion traps enable both storing ionized particles and separating the stored ionized particles according to their mass (M) to charge (Q) ratios. Storing the ionized particles involves applying a time-varying voltage to the ion trap so that particles propagate along stable trajectories therein. Separating the ionized particles typically involves applying an additional time-varying voltage to the trap so that the stored particles are selectively ejected according to their M/Q ratios. The ability to eject particles according to their M/Q ratios enables the use of ion traps as mass spectrometers.
Exemplary ion traps are described, e.g., in U.S. Pat. No. 2,939,952, issued Jun. 7, 1960 to W. Paul et al, which is incorporated herein by reference in its entirety.
FIG. 1 shows one type of quadrupole ion trap 10 that has an axially symmetric cavity 18. The ion trap 10 includes metallic top and bottom end cap electrodes 12–13 and a metallic central ring-shaped electrode 14 that is located between the end cap electrodes 12–13. Points on inner surfaces 15–17 of the electrodes 12–14 have transverse radial coordinates “r” and axial coordinates “z”. These coordinates satisfy hyperbolic equations, i.e., r2/r02−z2/z02=+1 for the central ring-shaped electrode 14 and r2/r02−z2/z02=−1 for the end cap electrodes 12–13. Here, 2r0 and 2z0 are the minimum transverse diameter and the minimum vertical height of the trapping cavity 18 that is formed by the inner surfaces 15–17. Typical trapping cavities 18 have a shape ratio, r0/z0, that satisfies: (r0/z0)2≈2, but the ratio may be smaller to compensate for the finite size of the electrodes 12–14. Typical cavities 18 have a size that is described by a value of r0 in the approximate range of about 0.707 centimeters (cm) to about 1.0 cm.
For the above-described electrode and cavity shapes, electrodes 12–14 produce an electric field with a quadrupole distribution inside trapping cavity 18. One way to produce such an electric field involves grounding the end cap electrodes 12–13 and applying a radio frequency (RF) voltage to the central ring-shaped electrode 14. In an RF electric field having a quadrupole distribution, ionized particles with small Q/M ratios will propagate along stable trajectories. To store particles in the trapping cavity 18, the cavity 18 is voltage-biased as described above, the particles are ionized, and then, the particles are introduced into the trapping cavity 18 via an entrance port 19 in top end cap electrode 12. During the introduction of the ionized particles, the trapping cavity 18 is maintained with a low background pressure, e.g., about 10−3 Torr, of helium (He) gas. Then, collisions between the background He atoms and ionized particles lower the particles' momenta thereby enabling trapping of such particles in the central region of the trapping cavity 18. To eject the trapped particles from the cavity 18, a small RF voltage may be applied to the bottom end cap 13 while ramping the small voltage so that stored particles are ejected through exit orifice 20 selectively according to their M/Q ratio.
For quadrupole ion trap 10, machining techniques are available for fabricating hyperbolic-shaped electrodes 12–14 out of base pieces of metal. Unfortunately, such machining techniques are often complex and costly due to the need for the hyperbolic-shaped inner surfaces 15–17. For that reason, other types of ion traps are desirable.
A second type of ion trap has a trapping cavity with a right circularly cylindrical shape. This trapping cavity is also formed by inner surfaces of two end cap electrodes and a central ring-shaped electrode located between the end cap electrodes. Here, the end cap electrodes have flat disk-shaped inner surfaces, and the ring-shaped electrode has a circularly cylindrical inner surface. For such a trapping cavity, applying a voltage to the central ring-shaped electrode while grounding the two end cap electrodes will create an electric field that does not have a pure quadrupole distribution. Nevertheless, a suitable choice of the trapping cavity's height to diameter ratio will reduce the magnitude of higher multipole contributions to the created electric field distribution. In particular, if the height to diameter ratio is between about 0.83 and 1.00, the octapole contribution to the field distribution is small, e.g., this contribution vanishes if the ratio is about 0.897. For such values of this shape ratio, the effects of higher multipole distribution are often small enough so that the cavity is able to trap and store ionized particles.
For this second type of ion trap, standard machining techniques are available to fabricate the electrodes from metal base pieces, because the electrodes have simple surfaces rather than the complex hyperbolic surfaces of the electrodes 12–14 of FIG. 1. For this reason, fabrication of this second type of ion trap is usually less complex and less expensive than is fabrication of quadrupole ion traps whose electrodes have hyperbolic-shaped inner surfaces.